1. Field of the Invention
This invention relates to an oxygen sensor for detecting an oxygen content in a gas to be measured, such as an exhaust gas from an internal combustion engine, particularly to an oxygen sensor having a heater including a bottomed cylinder of a solid electrolyte having porous platinum electrodes on the inner and the outer surfaces thereof, the bottom or closed end of the cylinder being to be exposed in a gas to be measured, a housing accommodating the cylinder for gas tightly separating the inside of the cylinder from the gas to be measured, a protective metal cover or protector enclosing the outer circumferential surface of the closed end of the cylinder of the solid electrolyte and having at least one gas passage hole for the gas to be measured, and a rod-shaped heater inserted in the interior of the cylinder of the solid electrolyte.
2. Description of the Prior Art
Heretofore, oxygen sensors have been known which detect oxygen concentrations of gases exhausted from internal combustion engines, such as automobile engines etc. by using oxygen ion-conductive solid electrolytes, such as zirconia etc., based on the principle of oxygen concentration cell, to control air/fuel (A/F) ratio of the internal combustion engines. Usually, this type of oxygen sensor is provided with a bottomed cylinder of a solid electrolyte made of zirconia, and porous platinum electrodes formed on the inner and the outer surfaces of the zirconia cylinder, the inner electrode being communicated with the exterior atmosphere to form a standard electrode of a standard oxygen concentration, and the outer electrode being exposed to a gas to be measured such as an exhaust gas from an automobile engine to form a measuring electrode, and measures an electromotive force resulted from a difference of oxygen concentrations between the standard electrode and the measuring electrode to determine an oxygen concentration in the gas to be measured.
However, the oxygen sensor does not generate a stable electromotive force, unless the solid electrolyte is heated to a sufficient extent. Therefore, it has a drawback in that the A/F ratio can not be controlled exactly at an idling state or immediate after the start of the internal combustion engine where the solid electrolyte of the oxygen sensor is still cold.
In order to solve the above drawback, various methods of forcibly heating the solid electrolyte have been proposed. For example, U.S. Pat. No. 4,155,827 discloses insertion of a heater composed of a heater wire wound on a rod-shaped insulator surface in the interior of the solid electrolyte, and U.S. Pat. No. 4,212,720 disclosed insertion of a so-called "sheath heater" composed of a metal sleeve and a resistive coil and a good thermally conductive and electrically non-conductive powder filled in the metal sleeve in the interior of the solid electrolyte.
However, this type of conventional oxygen concentration detectors or oxygen sensors having a heater have drawbacks in that when the temperature of the exhaust gas from an internal combustion engine becomes high, the solid electrolyte is suffered from excessive heating so that the porous platinum electrodes recrystallize to retard the rate of response to the exhaust gas, that a spinel coating layer protecting the porous platinum electrodes is peeled off or yields a crack, and that the heater per se becomes to an extraordinary high temperature due to heat generation thereof together with the heating thereof by the exhaust gas to break down the resistive wire therein.
If the amount of heat generated by the heater is suppressed to a small value for mitigating the above problems as far as possible, then another problems are incurred in that the solid electrolyte is insufficiently heated when the temperature of the exhaust gas is low, and that it takes a long time before the oxygen concentration detector generates an exact electromotive force, even when the heating is effected from the start of the engine.
Moreover, the battery voltage decreases at the time of starting the engine or at the time of cold climate, so that the problems of insufficient heating are even more accelerated. While, at the time of high revolution rate of the engine and elevated state of the battery voltage, the temperature of the exhaust gas becomes also high and the amount of heat generated by the heater is also increased, so that the problems of excessive heating are also even more accelerated.
Furthermore, a phenomenon of migration of silver occurs which results from a prolonged passage of an electric current through the brazing element containing the silver, so that a drawback also arises in that the brazed portions of the lead terminals of the heater are short-circuited therebetween.
In order to obviate these drawbacks, U.S. Pat. No. 4,512,871 (Japanese Utility Model Application Laid-Open No. 60-106,160) proposed an oxygen detector having a heater which comprises, a bottomed cylinder of a solid electrolyte 1 having porous platinum electrodes on the outer and the inner surfaces thereof as shown in FIG. 1, a rod-shaped heater 6 inserted in the interior of the solid electrolyte 1. The rod-shaped heater 6 is composed of a ceramic heater body 8, and a heating element 17 embedded in the ceramic heater body 8, lead portions 18 of the heating element 17 communicating the heating element 17 with the connecting portion 16, lead terminals 9, and connecting portions 16 for connecting the lead terminals 9 to the heater body 8, as shown in FIG. 2. Each connecting portion 16 is provided with a heater lead layers 15 communicated with the heater body 8, a metal plating layer 10 applied on the heater lead layer 15, the lead terminal 9 being fixed on the metal plating layer by brazing by a brazing element 11 containing silver, a metal coating layer 12 consisting of an electroless plating layer of metal other than silver, such as nickel, applied on the brazing element 11, as shown in FIG. 4.
As shown in FIG. 4, the lead terminal 9 consisting of a nickel wire is fixed to the metal layer 10 consisting of a plating layer of nickel by brazing by means of the brazing element 11, the metal layer 10 being applied on the ceramic heater body 8 made of alumina or the like via a heater lead layer 15 made of tungsten. Therefore, when using the oxygen sensor after assembled, the metal plating layer 10 which forms the brazing part of the lead terminal of the heater is influenced by temperature change of the exhaust gas of the internal combustion engine or temperature cycle caused by ON-OFF of the heater at the time of starting the engine or stopping the engine. Thus, a heat cycle is repeatedly imposed on the metal plating layer 10. As a result, there are hitherto problems that a crack occurs on the ceramic body 8 of the heater, and that the lead terminal 9 is disengaged from the heater body 8. Besides, the heater lead layer 15 made of tungsten can not be directly brazed by a brazing element, so that the metal plating layer 10 such as nickel plating has to be applied thereon for accomplishing the brazing.